Different PCB color (is it a different material?)

The green areas have been coated in solder-mask.
The yellowish area is simply the base color of the PCB material, which has not been coated.
It's the same base material throughout.

You would typically achieve this in your designs by adding features to the solder-mask layer(s) of your board.
The solder-mask layers are usually 'negative' images, so drawing something on those layers will result in the solder-mask being left out in those areas on your board.

The yellowish PCB laminate has better electrical insulation than some solder masks, so sometimes air gaps and solder mask free areas are used to separate the high voltage and low voltage side.

I work as CAM/CAM designer in a PCB fab, and recently we made a run of boards with no solder mask at all, because the boards were going in X-ray machines. Extremely high voltages, and the voltages would jump very easily through the solder mask.

The yellowish line in the middle of the board is the true PCB colour.

The green areas either side are solder resist.